The present invention relates to a means for detecting infrared radiation and analyzing materials thereby.
In general, infrared analyzers can be classified into two groups, double-beam type infrared analyzers and single-beam type infrared analyzers. FIG. 2 shows the arrangement of a typical double-beam infrared gas analyzer, and FIG. 3 shows the arrangement of a typical single-beam infrared gas analyzer. As conducive to a full understanding of this invention, the operating principle of an infrared gas analyzer will be described with reference to FIGS. 2 and 3.
As shown in FIG. 2, a light source 2 is provided in a light source chamber 1. The light source 2 emits infrared rays. The infrared rays are divided by a partition chamber 3 into a measurement beam IM and a reference beam IV which are equal in the amount of light in the beam (hereafter light flux). The measurement beam IM is sent into a measurement cell 4, and the reference beam IV is sent into a reference cell 5. A material to be measured, namely, a gas 100 which contains a first component is introduced through a conduit 6 into the measurement cell 4. On the other hand, a gas such as nitrogen gas having no infrared ray absorbing characteristic is sealed in the reference cell 5. The measurement beam IM is subjected to absorption by the first component, and therefore its optical intensity is decreased in accordance with the density of the first component. On the other hand, the optical intensity of the reference beam IV is maintained unchanged, because the reference beam IV is not absorbed. The measurement beam IM and the reference beam IV which have passed through the measurement cell 4 and the reference cell 5, respectively, are applied to gas-sealed detector 8. The second detecting chamber 10 is filled with the gas which is the same as the first component. The measurement beam IM is applied to the first detecting chamber 9, while the reference beam IV is applied to the second detecting chamber 10 which is filled with the gas which is the same as the first component. The gas in the first detecting chamber 9, and the gas in the second detecting chamber 10 absorb the measurement beam IM and the reference beam IV, respectively. Therefore, the gases in the first and second detecting chambers 9 and 10 are heated to different temperatures according to the intensities of the measurement beam IM and the reference beam IV, respectively. The detecting chambers 9 and 10 communicate with each other through a conduit 11. Wire heating elements 12 and 13 are arranged at the middle of the conduit 11 in such a manner that they are thermally coupled to each other. The elements 12 and 13 together with two external resistors (not shown) form a bridge circuit, and are heated by a power source to a temperature higher than the ambient temperature. As the gases in the detecting chambers 9 and 10 are heated by the measurement beam IM and the reference beam IV, respectively, the gases in the detecting chambers 9 and 10 expand, as a result of which a gas flow occurs in the conduit 11 with a flow rate corresponding to the density of the first component in the gas 100. The flow rate is converted into an electrical signal by the elements 12 and 13. A rotor 14 is provided between the detector 8 and cells 4 and 5. The rotor 14 is rotated by a motor M, to periodically intercept the measurement beam IM and the reference beam IV which are applied to the detector 8. FIG. 2 depicts a trimmer 15 for adjusting the light quantities of the beams IM and IV so that they are equal to each other at all times; and 16, 17, 18, 19, 21, 22, and 23 are light transmitting windows.
An amount of optical energy .DELTA.I.sub.1 absorbed in the first detecting chamber 9, and an amount of optical energy .DELTA.I.sub.2 absorbed in the second detecting chamber 10 can be expressed by the following equations (1) and (2), respectively: EQU .DELTA.I.sub.1 =IMexp(-.alpha.CL){1-exp(-.alpha.C.sub.O L.sub.1)}(1) EQU .DELTA.I.sub.2 =Ir.multidot.{1-exp(-.alpha.C.sub.O L.sub.1)}(2)
Where L is the length of the measurement cell, L.sub.1 is the optical path length of the measurement cell, .alpha. is the absorption coefficient of the first component gas, C.sub.O is the density of the gas sealed in the detector; C is the first component gas density, Im is the measurement cell incident light flux and Ir is the reference cell incident light flux.
The pressure increments .DELTA.P.sub.1 and .DELTA.P.sub.2 of the first and second detecting chambers 9 and 10 are proportional to .DELTA.I.sub.1 /V and .DELTA.I.sub.2 /V (where V is the volume of each of the first and second detecting chambers 9 and 10).
Therefore, EQU .DELTA.P.sub.2 -.DELTA.P.sub.1 .varies.Ir-Im.multidot.exp(-.alpha.CL) (3)
The trimmer 15 is operated to adjust the light quantities so that Im=Ir=I.sub.O , then EQU .DELTA.P.sub.2 -.DELTA.P.sub.1 .varies.I.sub.O -I.sub.O .multidot.exp(-.alpha.CL).apprxeq..lambda.CLI.sub.0 ( 4)
Therefore, the velocity v of the gas flow in the conduit 11 is: EQU v.varies.(.DELTA.P.sub.2 -.DELTA.P.sub.1).varies. C (5)
Thus, with the aid of the heat wire elements 12 and 13, an electrical signal E proportional to the gas density C can be obtained.
Now, the arrangement and the operating principle of the single-beam type infrared analyzer will be described with reference to FIG. 3. In FIG. 3, parts corresponding functionally to those which have been described with reference to FIG. 2 are designated by the same reference numerals or characters. The measurement beam IM emitted by light source 2 is partially absorbed by the first component gas in the measurement cell 4, and reaches a detector 25. The detector 25 comprises: a first detecting chamber (front chamber) 26 and a second detecting chamber (rear chamber) 27 which are arranged in the direction of the optical path of the measurement beam IM and filled with the first component gas; and a passage 28. That is, the detector is a serial-double-chamber type detector in which the measurement beam Im is partially absorbed in the first detecting chamber 26 and then absorbed in the second detecting chamber 27. The difference between the pressure increments which are caused by the absorption of the measurement beam in the first and second detecting chambers 26 and 27 is converted into an electrical signal by the heat wire elements 12 and 13 provided in the passage 28. In FIG. 3, reference numerals 16, 19, 21, 29 and 30 designate light transmitting windows.
It is assumed that the optical path lengths of the first and second detecting chambers 26 and 27 are represented by L.sub.2 and L.sub.3 respectively, the volumes of the chambers 26 and 27 are by V.sub.1 and V.sub.2 respectively, the density of the gas sealed in the detector is represented by C.sub.O the measurement cell incident light flux is represented by I.sub.O the measurement cell length is represented by L, the measurement cell transmission factor of the measurement beam IM which depends on the degree of any light absorbant coating (e.g., dust, soot etc.) of the inner wall of the measurement cell 4 is represented by T, and the density of the first component gas is represented by C. Then, the amounts of optical energy .DELTA.I.sub.2 and .DELTA.I.sub.2 which are absorbed in the first and second detecting chambers 26 and 27 are as follows: ##EQU1##
The pressure increments .DELTA.P.sub.1 and .DELTA.P.sub.2 in the first and second detecting chambers are: ##EQU2## where K is the constant.
Therefore, the velocity v of the gas flow in the passage 28 is: ##EQU3##
Thus, with the aid of the elements 12 and 13, an electrical signal corresponding to the density C of the first component gas can be obtained.
As is apparent from the expressions (4) and (9), in the conventional gas analyzers shown in FIGS. 2 and 3, the zero point and the span point change with intensity of the measurement beam IM. Therefore, conventional gas analyzers suffer from a difficulty that measurement errors are caused by dust included in the gas 100 to be measured or by dust on the wall 31 of the measurement cell 4 or the light transmitting windows 19 and 21. In order to eliminate this difficulty, conventional analyzers employ a gas sampling system in which, when a gas 100 to be measured is introduced into the measurement cell from a measurement point such as a flue, a dust filter or the like is used to remove dust from the gas. That is, heretofore, a conventional gas analyzing system employed a gas sampling system so that the measurement would not be significantly affected by the dust in the gas being measured. However, such a method is still disadvantageous because of the following features: It is necessary to periodically replace the filter or to periodically calibrate the instruments; that is, it is necessary to perform maintenance for the analyzer periodically. Furthermore, because it takes a relatively long time to lead the gas to be measured from the measurement point to the measurement cell 4, the response speed of the analyzer is low. Therefore, the analyzer is not applicable to a technical field such as combustion control in which a high speed response is essential. On the other hand, a gas filter correlation type (GFC type) infrared analyzer and an infrared dual-channel type analyzer have been proposed as infrared analyzers which are not affected by variations in the light flux. However, the GFC type infrared analyzers is low in both durability and in reliability, because it is necessary to turn the gas filter at high speed. The infrared dual-channel type analyzer is also disadvantageous in that it is expensive because an infrared tunable laser is employed as its light source.
Accordingly, it is an object of this invention to provide an infrared analyzer in which the above-described difficulties accompanying a conventional infrared analyzer have been eliminated, and which is applicable to combustion control or the like because of it being sufficiently high in response speed. An additional object of the present invention is to be low in operation cost and free of maintenance. A further object of the invention is to provide an infrared analyzer in which the measurement result is not affected by coatings on the interior measurement cell due to solids in the material under measurement which contains the particular component to be measured. In addition, it is a further object of the invention that the analyzer neither be affected by the second component which coexists with the particular component in the material under measurement and has an infrared ray absorbing wavelength band different from that of the particular component.